The present invention relates to the field of papermaking and, more specifically, to the production of paperboard. Still more specifically, the present invention relates to use of wet-end additives to the furnish or stock.
In the manufacture of paper and paperboard, an aqueous slurry of cellulosic fibers is prepared and delivered to a draining wire screen which results in the formation of a mat on the screen and the draining of an aqueous suspension known as white water through the screen. The white water is subsequently recycled. The aqueous cellulosic suspension or slurry that is delivered to the screen is known as the thin stock.
The amount of cellulosic fibers in the thin stock typically ranges from about 0.5% to about 1%. The thin stock is typically prepared from another aqueous solution or slurry of cellulosic fibers known as the thick stock that has been diluted with recycled white water. The thin stock also includes a number of different additives which are introduced to the system after the dilution of the thick stock with the white water. The amount of cellulosic fibers in the thick stock typically is about 3% while the amount of cellulosic fibers in the white water is typically less than 1%.
By way of example only, a simplified and abbreviated flow sheet of one paper or paperboard making process is shown in FIG. 1. It will be noted that FIG. 1 illustrates just one of many different paper and board making processes to which the present invention applies. the use of FIG. 1 for illustration purposes is not intended to limit the methods of the present invention to the processes illustrated in FIG. 1. Rather, the inventive methods can be used to improve board and papermaking processes for dual former machines, gap former machines and other machines in addition to the Fourdrinier processes illustrated in FIG. 1. The system 10 includes a head box 11 and a wire screen shown at 12 which is an endless wire screen on which the thin stock is fed. FIG. 1 represents one design known as a fourdrinier paper machine. Similar principles apply to other paper making machine designs. The head box 11 receives the thin stock from the line 13 and delivers the thin stock slurry to the wire 12. As a result, a mat is formed on the wire 12 and finally transported out of the system as a sheet 14.
The bulk of the water from the thin stock is drained from the mat that is formed on the wire 12 and is recycled through the line 15 as white water. The white water is collected in the white water silo 16 where it is recycled mainly to a primary pump shown at 17.
In the system 10 shown in FIG. 1, the pump 17 includes two inlets 18, 19. The inlet 18 receives white water flowing through the line 21 from the white water silo 16. The inlet 19 receives thick stock flowing through the line 22 from the machine chest 23. The pump 17 mixes the white water and thick stock 19 to produce a thin stock which is pumped through the line 24 finally into a pressure screen shown at 25. The combination of the action of the pump 17, pressure screen 25 and additional shear imposing components such as a vortex cleaner (not shown) in combination with additional pressure screens results in a thorough mixing of the thin stock.
In the production of any high quality paper or paperboard, three characteristics are required: drainage; retention; and formation.
First, the liquid components of the thin stock must drain well from the wire 12 so that a mat or sheet having a low water content is formed on the wire 12 and produced at 14 for drying.
Second, thin stock also includes a number of additives and fines which are small particles of fiber that are shorter than normal wood pulp fibers. In order to produce paper efficiently and in order to produce paper that uses less cellulosic fibers, it is extremely important that the mat or sheet retain the cellulosic fines and other additives that are suspended in the thin stock. Thus, in addition to the importance of drainage to the paper formation process, it is also important to effectively retain additives, fillers and fines in the mat.
Third, formation is a measure of the uniformity of the paper sheet and is generally determined by variances in the transmission of light through a paper sheet, high variance being indicative of poor formation and poor paper quality.
In order to increase retention, additives are added to either the thin stock or thick stock in the form of coagulants and flocculants. Specifically, coagulants are low molecular weight cationic synthetic polymers or cationic starches which generally reduce the negative surface charges present on the mineral fillers and cellulosic fines present in the thin stock which results in an agglomeration of the particles. The agglomeration of the particles assists in the retention of the particles in the web or sheet.
Additionally, flocculants are utilized which are generally high molecular weight anionic synthetic polymers which bridge the agglomerated particles from one surface to another thereby binding the particles into larger agglomerates. The presence of these larger agglomerates in the thin stock increases retention further. The larger agglomerates are better retained in the mat.
However, the use of retention aides such as coagulants and flocculants has the tendency of compromising formation properties in the paper because large agglomerates tend to contribute to nonuniformity of the mat and therefore the finished paper sheet or board. Hence, if the flocculation, especially homoflocculation of fiber, is increased to an excessive degree, formation will be compromised and the quality of the resulting product will suffer.
One particular filler or additive which has been used in the papermaking industry to a large degree is bentonite. Bentonite is a naturally occurring clay comprising minerals that swells and forms a colloidal aqueous suspension. Bentonite is used as a pitch remover in papermaking water systems, as well as a retention additive, a microparticle and as a filling agent. The use of bentonite in the presence of other retention aides such as coagulants and flocculants requires certain considerations.
Specifically, because bentonite has flocculating properties, there is a danger that the combination of bentonite and a flocculant or coagulant will result in excessively large agglomerates which will adversely affect the formation qualities of the paper. As a result, bentonite is often added after the thin stock has passed through the primary pump 17 and pressure screen 25 (as well as any additional shear imposing components such as vortex cleaners) and just before the thin stock enters the head box 11. By adding bentonite after the thin stock is treated with coagulants and flocculants and mixed in the pump 17 and is passed through the pressure screen 25 as well as any other shear imposing components such as vortex cleaners (not shown), it is believed that the creation of large bentonite containing agglomerates can be controlled.
There is a need for an improved papermaking and paperboard making process utilizing bentonite which provides the paper manufacturer with greater flexibility as to where the bentonite is added to the system. An improved process would allow papermakers to better balance the performance by allowing for increase of retention and drainage while not adversely affecting formation.